Transcript F324 summary

F324 summary
Benzene
• Benzene, C6H6, consists of a sigma-bonded framework
of carbon and hydrogen atoms.
• Above and below the plane of atoms is a p-bond,
which consists of a delocalised electron cloud.
• The Kekule structure of benzene assumes that all the
bonds are localised i.e. cannot move. However,
evidence to support the delocalised form of benzene
comes from bond lengths, enthalpy change of
hydrogenation and resistance to reaction.
• Electrophiles (accept lone pair of electrons) can attack
a benzene ring – usually resulting in an electrophilic
substitution process as a mechanism for a reaction.
Reactions of Benzene
• Benzene’s chemical reactions tend to have a high
activation energy due to the delocalised p-electron
system.
• Benzene can react with the following:
i. concentrated nitric acid in the presence of
concentrated sulfuric acid to form nitrobenzene
ii. a halogen in the presence of a halogen carrier to form
a mono-halogenated benzene compound e.g.
chlorobenzene or bromobenzene. This reaction is
slower than with cyclohexene due to the enhanced
thermodynamic stability provided by the delocalised
p electron system in benzene.
Phenols
• Phenol has the molecular formula C6H5OH. It is used in
the production of plastics, antiseptics, disinfectants
and resins for paints.
• Phenol reacts with:
• alkalis to form sodium phenoxide (a salt) and water
• sodium to form sodium phenoxide and hydrogen.
• Phenol reacts rapidly with bromine to form 2,4,6tribromophenol and hydrogen bromide. This reaction is
faster than with benzene because the oxygen lone pair
of electrons in phenol overlaps with the p system on
the ring, increasing its electron density and facilitating
electrophilic attack.
Carbonyl compounds
• Carbonyl compounds include aldehydes and ketones.
• Aldehydes can be oxidised in the presence of acidified
potassium dichromate(VI) to form carboxylic acids. The
orange dichromate(VI) ion is reduced to the green
chromium(III) ion.
• Ketones cannot be oxidised under these conditions.
• Aldehydes can be reduced using sodium borohydride
to form primary alcohols.
• Ketones can be reduced using sodium borohydride to
form secondary alcohols.
Tests for carbonyl compounds
• 2,4-dinitrophenylhydrazine (2,4-DNP/2,4-DNPH/ Brady’s
reagent) is used to detect the presence of the >C=O group
in aldehydes and ketones – if present, an orange precipitate
is formed.
• The precipitate may be recrystallised in ethanol and its
melting point measured – this information may then be
used to determine the identity of the original molecule.
• Tollens’ reagent (ammoniacal silver(I) nitrate) is used to
detect the presence of an aldehyde group. A silver mirror is
formed on warming the aldehyde with Tollens’ reagent and
a carboxylic acid is also formed. There is no reaction with
ketones.
Carboxylic acids and esters
• Carboxylic acids contain the –COOH group. This group
is highly polar, and that is why carboxylic acids:
i. have higher melting and boiling points than expected
– hydrogen bonding between molecules
ii. are water-soluble – hydrogen bonding between
carboxylic acid and water molecules.
• Carboxylic acids are weak acids and will react with
reactive metals, metal oxides (and hydroxides) and
metal carbonates to form carboxylate salts containing
the –COO– ion.
Reactions of carboxylic acids
• Carboxylic acids react reversibly with alcohols, in the
presence of an acid catalyst, to form esters – used in
perfumes and flavourings.
• Esters may be hydrolysed in the presence of:
• hot alkalis to form the corresponding alcohol and the
carboxylate salt
• hot acids to form the corresponding alcohol and the
carboxylic acid.
• Fats and oils are naturally occurring esters. The fatty acid
part of the molecule may be either:
• saturated or
• unsaturated – can be either cis or trans.
Amines
• Amines are molecules containing a nitrogen atom –e.g.
the primary amine functional group –NH2.
• Amines are bases since they can accept a proton by
using the lone pair of electrons on the nitrogen atom.
• Amines react with acids to form salts.
• Aliphatic amines may be prepared by the substitution
of halogenoalkanes with excess ammonia.
• Aromatic amines can be prepared by reducing
nitroarenes with tin and concentrated hydrochloric
acid.
• Azo dyes are made by reacting an aromatic amine with
nitrous acid and then phenol in alkali.
Amino acids
• The general formula for an a-amino acid is
RCH(NH2)COOH.
• As an amino acid has both an acidic group (–COOH)
and a basic group (–NH2), it can act as both an acid
(proton donor) and a base (proton acceptor).
• At a certain pH known as the isoelectric point, a
zwitterion forms. This contains the –NH3+ group and
the –COO– group in the same molecule.
• Amino acids polymerise to form proteins while proteins
can undergo hydrolysis to form a-amino acids.
Optical isomerism
• Optical isomers are non-superimposible mirror images
about an organic chiral centre – four different groups
attached to a carbon atom.
• When a chiral molecule is synthesised in the
laboratory, usually many optical isomers can form.
• However, if an enzyme is involved in the synthesis,
normally only one optical isomer forms.
• If pharmaceutical products contain only one optical
isomer the possibility of side effects is reduced and the
pharmacological activity is improved.
• Separating optical isomers is expensive, so chiral
synthesis producing just one isomer is better
Condensation polymerisation
• Examples of condensation polymers are polyesters
(e.g. Terylene) and polyamides (e.g. nylon-6,6).
• When a condensation polymer is formed, a small
molecule such as water or hydrogen chloride is also
formed.
• Terylene is made from benzene-1,4-dicarboxylic acid
and ethane-1,2-diol.
• Nylon-6,6 is made from 1,6-diaminohexane and
hexane-1,6-dicarboxylic acid.
• Kevlar is a special type of nylon that is made from
benzene-1,4-diamine and benzene-1,4-dicarboxylic
acid.
Hydrolysis and degradable polymers
• Condensation polymers have chemical groups that are
vulnerable to chemical attack from either acids or
alkalis – polyesters (ester group) and polyamides
(amide group). This process is known as hydrolysis and
results in the breakdown of the polymer.
• Disposing of polymers is an environmental problem.
Scientists are working to develop degradable polymers
similar in structure to poly(lactic acid).
• Condensation polymers may photodegrade as the C=O
bond absorbs radiation
Synthetic routes
• Novel, useful molecules can be synthesised using
organic chemistry.
• A chiral molecule is more difficult to synthesise since
many other optical isomers may also form – costly in
money and time to separate or resolve the isomers.
• Enzymes, bacteria, chiral catalysts and chiral-starting
points (e.g. L-amino acids) are used to promote the
formation of one chiral product.
• The chemical reactions you study during your A level
chemistry course may be used to suggest how a target
molecule can be synthesised, starting with a particular
organic molecule.
Types of chromatography
• Chromatography is an analytical technique that separates
components in a mixture between a mobile phase and a
stationary phase.
• The mobile phase may be a liquid or a gas.
• The solid phase may be:
a. a solid (as in thin-layer chromatography(TLC)) or
b. a liquid or a solid on a solid support (as in gas
chromatography (GC)).
• The Rf value measures the ratio of the distance moved by
the solute to the distance moved by the solvent.
• GC does have limitations – e.g. similar compounds have
similar retention times.
Combining mass spectrometry with
chromatography
• Mass spectrometry (MS) can be combined with
chromatography:
a) to provide a far more powerful analytical tool
than using chromatography alone
b) to generate mass spectra that can be analysed
or compared with a spectral database by a
computer for positive identification of a
component.
• GC–MS can be used in analysis (e.g. in forensics),
environmental analysis, airport security and
space probes.
Proton NMR
• In NMR, protons (hydrogen atoms) in a sample absorb
and emit low-energy radiowave radiation in the
presence of a powerful magnetic field.
• The number of peaks gives information about the
number of proton environments.
• The area under each peak gives information about the
number of hydrogen atoms in each environment.
• Their horizontal position in a spectrum gives the
chemical nature of each proton region.
• Protons that are adjacent to unequivalent protons may
be split into more peaks, where n+1 gives the number
of splits (if adjacent to n hydrogen atoms).
Carbon-13 NMR spectroscopy
• The number of peaks in a carbon-13 spectrum is the
same as the number of different carbon environments.
• The relative position of each peak on the horizontal
axis (the chemical shift) suggests the chemical
environment of a particular carbon atom.
• This means it is possible from the carbon-13 spectrum
alone to suggest the likely structure of a molecule – by
counting the number of peaks and deducing the
chemical nature of the bonded atoms to each carbon.